1. Field of the Invention
The present invention relates to a hydraulic shift transmission for vehicles. Particularly, it relates to a hydraulic shift transmission for a vehicle including a hydraulic torque converter for transmitting the engine torque to a road wheel driving system.
2. Description of Relevant Art
In the art of vehicles in which engine torque is transmitted through a hydraulic torque converter to a road wheel driving system, there have been employed a variety of hydraulic shift transmissions including a plurality of gear trains interposed between the hydraulic torque converter and the road wheel driving system. The gear trains generally have their ratios different from one another. A plurality of clutch mechanisms are alternatively installed in the gear trains. The clutch mechanisms each are hydraulically connectable and disconnectable. A hydraulic system is provided for supplying hydraulic oil to the clutch mechanisms in accordance with shift operations.
In conventional hydraulic shift transmissions of such type, as an object to be solved there has been a problem of what is called "creep", that is, in the state where a shift lever provided in the vicinity of driver's seat is placed in its advance position such as when waiting for a signal or in a traffic snarl, even while the vehicle is stopped, the torque of an engine idling may be transmitted to road wheels, thus causing the vehicle to advance opposing the driver's intention.
This "creep" has been avoided by footing a braking pedal, thus needing the driver to be alert and to intentionally keep a firm footing on the braking pedal. Additionally, the engine throttle valve is kept excessively open for the stirring torque of a hydraulic converter, thereby raising the fuel consumption rate. This results in a disadvantage from the viewpoint of energy saving.
On this account, there have been provided a number of appreciable proposals for solving these problems such as an interruption of the hydraulic oil supply to a first speed clutch as the vehicle is stopped with the engine idling, as disclosed by the Japanese Patent Publication No. 49-5772, for example.
Yet, for the employment of such proposal, new proposals have developed such as that, when changing the foot onto the acceleration pedal to start the vehicle, a starting shock is to be felt, accompanying such disadvantages that, for improvements in this respect, it has been hardly successful to rely on conventional measures without introducing a large-sized or complex device.
The present invention has been achieved to effectively solve such problems and disadvantages in conventional hydraulic shift transmissions.